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FLUXING AND SLAGGING OUT FURNACES
The percentage of ore and fuel which must be carried off by the slag" in making iron consists of ten to thirty per cent of the former and ten to fifteen per cent of the latter. A portion of this extraneous matter is basic, the rest acid. The chemical affinity thus existing" is such that, when this material is subjected to high heat, union is effected, the whole passing into a fluid state. Generally the percentage of basic in the refuse is not sufficient in its action on the acid matter to reduce it to such a fluid state that it will flow freely, or properly extract all extraneous matter from the ore. To remedy this defect, limestone or other flux is generally added to all charges of ore going to a furnace. While the lime, etc., assists in fluxing the refuse to the state of fluidity required, it also affects the quality of the iron produced as described in pages 53 and 54.
The grade of iron which is to come from a furnace can generally be foretold by the nature of the slag tapped or flushed before the iron is tapped. If a lump of solid slag, when broken, presents a black color, very dense in its composition, it is generally supposed to denote the production of iron very low in silicon and high in sulphur, with high iron in the slag. If slag is of a light or gray color and its fracture presents a poroushe iron and raises its sulphur contents. For cupola work preference, as far as labor is concerned, would be given to Nos. 2 and 3 owing to these being more friable than No. i, but the furnace limestone No, i is will also show that a flux which, might work well in a furnace can often be well utilized in cupola practice:gases, heated stock, and liquid metal should enable can counteract the absorbing power ofee reactiony they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are if j